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Epsilon Eridani - A Norse God in the River

A great river flows through the southern sky, the ancient constellation Eridanus (the River). One of its stars is Epsilon Eridani, a near neighbor in space. The star is of special interest to us, both because of its closeness and because this sunlike star has a planetary system. The star and its known planet were given official names in 2015 by the International Astronomical Union (IAU) in consultation with the public.

Our neighbor, but for how long?
In the 19th century astronomers were trying to calculate the distances to stars. They knew that they should be able to do this by using parallax, the triangulation that surveyors use on Earth. This diagram shows how stellar parallax works using a bit of geometry. The parallax angle is labelled "p", and you can see that the angle would be smaller for a more distant star.

Although nearer stars have bigger parallaxes, all the stars we see are a long way off. The parallax angle of the closest one is only a fraction of a degree. However even in the 18th century, it was still possible to measure the angles for some stars. Epsilon Eridani was one of them. It has a high proper motion, which shows that it's close to us. Proper motion is a star's apparent motion across the sky relative to more distant stars. Measurements showed Epsilon Eridani to be about ten light years away.

Interestingly, the star used to be closer to us – it was only seven light years distant 105,000 years ago. Proper motion is only apparent motion, but Epsilon Eridani is also on the go. It may well be a member of the Ursa Major Moving Group. This consists of a number of stars of similar age moving in the same direction at about the same velocity. They probably all formed in one giant cloud and were once part of the same star cluster before it gradually broke up.

The star
Although Epsilon Eridani is not particularly bright, it's visible from almost everywhere on Earth. The astronomer Ptolemy included it in his star catalog Almagest in the second century, and it has shown up in star catalogs over the many centuries since then.

Astronomers can't be precise about the star's age, but estimates are generally from 200-800 million years. It's definitely a youngster. (Our Sun is about 5 billion years old.) Comparing it to the Sun, Epsilon Eridani is somewhat smaller, with a radius three-quarters that of the Sun and 82% of its mass. It's cooler than the Sun – 5000 °C to the Sun's 6000 °C – and is only about a third as luminous.

The young star has more magnetic and sunspot activity than the Sun, so its luminosity is more variable. It spins on its axis about twice as fast, and its stellar wind is thirty times stronger than the Sun's. That means it's sending out a lot of charged particles.

E.T., are you there?
Some astronomers have been searching for signs of intelligent aliens. Project Ozma was the first such search. Frank Drake started it in 1960 and used a radio telescope to observe Epsilon Eridani and Tau Ceti. Like Epsilon Eridani, Tau Ceti is a nearby sunlike star, and it seemed reasonable that they might have inhabited planets. However in the 150 hours of data stored on tape over a 4-month period, nothing suggested intelligence.

A planetary system
Project Ozma was long before the first exoplanets were discovered in 1995. We now have evidence of planets orbiting both Epsilon Eridani and Tau Ceti, but we also know that Epsilon Eridani is quite a young star. We don't know if it has a planet or moon with primitive life, but it's very unlikely to have any life that's communicating with radio signals.

In this diagram the Epsilon Eridani system is compared to the Solar System. There is a giant planet larger than Jupiter (Epsilon Eridani b), two asteroid belts, and a belt of comets. Some researchers also suggest that a second planet – Epsilon Eridani c – would explain some of the features of the inner asteroid belt.

But is there a terrestrial planet in the star's habitable zone? The habitable zone is the region where liquid water could persist on the surface of a planet or moon. You need other conditions too, but water is the starting point. The habitable zone for Epsilon Eridani is now about 0.5 – 1.0 AU out from the star. (An AU is the Earth-Sun distance, i.e., 150 million km or 93 million miles.) However, as the young star becomes more luminous, the zone will move outward.

Naming names
The naming convention for exoplanets, used by the International Astronomical Union (IAU), is the star name plus “b” for the first planet discovered. Then they work through the alphabet for any others. But in response to public interest, in 2015 the IAU released names for some stars and exoplanets. The names had been proposed by various organizations and voted on by the public.

Epsilon Eridani and Epsilon Eridani b were on the IAU list, and have acquired the names of Norse gods. The star is Ran, a sea goddess “who stirs up the waves and captures sailors with her net.” The planet is AEgir [EYE.geer], Ran's husband, “the personified god of the ocean.”